The present invention relates to a radiological imaging apparatus and a timing correction method therefor, and more particularly, to a radiological imaging apparatus and a timing correction method therefor suitable for use in a Positron Emission Tomography (hereinafter referred to as “PET”) apparatus.
A PET inspection is an inspection carried out by administering radio pharmaceuticals (hereinafter referred to as “PET pharmaceuticals”) containing positron emitters (15O, 13N, 11C, 18F, etc.) and having the nature of accumulating in a specific region (e.g., cancer cells) to an examinee and detecting γ-rays emitted from the affected area of the examinee by being provoked by the PET pharmaceuticals accumulated in the region using radiation detectors. When a positron emitted from the positron emitter contained in the PET pharmaceuticals encounters with neighboring electrons and annihilates, a pair of γ-rays having energy of 511 keV are emitted in substantially diametrically opposite directions. It is possible to identify locations where the PET pharmaceuticals are accumulated, that is, the affected area of cancer of the examinee based on the respective detection signals outputted from a pair of radiation detectors which have detected this pair of γ-rays.
To identify the affected area of cancer, it is necessary to identify the respective positions of the pairs of radiation detectors which have detected the pairs of γ-rays generated by annihilation of positrons and it is necessary to take a coincidence count of detection signals outputted from these radiation detectors. This requires time resolution with high precision. However, even when γ-rays enter two radiation detectors simultaneously, there is a variation in signal transmission from the respective radiation detectors to a coincidence circuit and there are differences in the times at which signals arrive at the coincidence circuit. For this reason, it is necessary to adjust transmission delays of signals from the respective radiation detectors so that the times at which signals arrive at the coincidence circuit coincide with one another.
Conventionally, timing correction of signals detected by radiation detectors is realized by acquiring calibration data using a calibration radiation source and adjusting a time variation of signal transmission based on the calibration data. This timing correction method is described, for example, in JP-B-6-19436 and Japanese Patent No. 3343122.
The signal timing correction method described in JP-B-6-19436 will be explained. First, γ-rays from the radiation source are detected by a radiation detector, a timing signal is created based on an output signal of the radiation detector and this timing signal is inputted to the coincidence count apparatus through a delay adjusting circuit. The sensitivity of the signal outputted from the coincidence count apparatus is measured. Next, the sensitivity of γ-rays from the radiation detector is measured using the same method as that described above while changing an amount of delay to be set in the delay time adjusting circuit. This is the method of correcting a signal delay time by setting the amount of delay corresponding to the highest measured sensitivity in the delay time control apparatus.
Next, the signal timing correction method described in Japanese Patent No. 3343122 will be explained. This method corrects timings of signal transmission by setting a calibration radiation source within a field of view of the PET apparatus, creating timing data indicating a time difference measured value of a coincidence event which occurs between a pair of radiation detectors, calculating a time delay value corresponding to each radiation detector and setting this time delay value in the corresponding radiation detector channel.
However, the above described conventional technologies obtain calibration data necessary for timing correction using a radiation source and γ-rays emitted for one event form a pair, and therefore it is possible to obtain calibration data for only the circuits connected to the two radiation detectors into which the respective γ-rays are introduced. For this reason, it takes a long time to obtain calibration data corresponding to all radiation detectors.
It is an object of the present invention to provide a radiological imaging apparatus and a timing correction method therefor capable of reducing a time required to acquire timing calibration data to be used for timing correction of output signals of radiation detectors.